A study of the reactions of free radicals with important substrates in biological systems is proposed. This study is designed to clarify the molecular mechanism by which radicals participate in natural processes such as aging or induced processes such as exposure to radiation or chemical carcinogens. It will be conducted on model systems and should give results which are applicable to higher levels of biological complexity. Specific studies include: An evaluation of the mechanism of free radical attack on biologically important thiols via radiolysis techniques, kinetics and mechanisms of thiyl radical reactions in model biological systems using a flash photolysis technique, and a study of secondary radical reactions with nucleic acid bases which includes the development of a sensitive gas chromatographic analysis technique.